A crosslinked polyethylene resin currently and mainly used as an insulating material of a power cable is a thermosetting resin to have not only excellent heat resistance, chemical resistance, and the like, but also excellent electric property. A scheme of crosslinking polyethylene includes a cross-linkage by a chemical reaction using an organic peroxide or silane as a mediator (U.S. Pat. No. 6,284,178 (Sep. 4, 2011), a radiation cross-linkage (U.S. Pat. No. 4,426,497 (Jan. 17, 1984), and the like, and recently, a crosslinked polyethylene resin using organic peroxide has been the most widely used in cable industries.
However, since the crosslinked polyethylene resin is a thermosetting resin, it is not recyclable to have difficulty in disposal, causing environmental pollution. Therefore, a non-crosslinked thermoplastic polyethylene resin which is environmentally friendly is required to be used; however, heat resistance is remarkably unsufficient, as compared to the crosslinked polyethylene resin, such that the non-crosslinked thermoplastic polyethylene resin has a limitation in being used as an insulator for a power cable.
A cross-linkage process is necessary for a process producing a power cable of crosslinked polyethylene using an organic peroxide, wherein in the cross-linkage process, high pressure and high temperature are required to be performed, productivity is remarkably decreased, and a difference in a degree of cross-linkage may occur by even small changes in processing conditions, such that uniformity of a product may be decreased.
In addition, during the cross-linkage process, an organic peroxide is decomposited by high temperature heat to produce radicals, thereby completing the cross-linkage reaction, wherein cumyl alcohol, methane, and the like, are generated as by-product of the cross-linkage reaction, to form bubbles in the insulator. To remove the bubbles, high pressure of 5 atm or more should be applied, and non-removed bubbles may be a material causing breakage of the insulator for a power cable.
Regarding this, Korean Patent Laid-Open Publication No. KR 10-2010-0106871 (Oct. 4, 2010) discloses a non-crosslinked polyethylene resin as an insulating material for a power cable; however, has a problem in that at the time of performing an actual process, processability is defective due to low shear thining of the resin, such that processability defect occurs. In addition, tracking resistance is defective, thereby causing a problem in that performance as an insulator of a cable for outdoors is deteriorated.
In order to solve the problem, a metal hydroxide is applied to the existing insulating resin composition (Korean Patent No. 10-0556318); however, the metal hydroxide is contained in a content of 10 to 30 parts by weight, thereby causing the following problems.
The metal hydroxide has poor compatibility with polyethylene, thereby having defective mechanical physical properties such as tensile strength, elongation retention, and the like. In addition, the metal hydroxide increases a density of a covering material of a cable, thereby causing a problem that the cable is slack due to the increased weight of the cable at the time of being installed, whereby life span becomes decreased or a distance between pylons should be decreased. Further, environmental durability may be deteriorated and pollutants may occur on a surface due to the metal hydroxide. Accordingly, hydrophilic property is increased to generate a leakage current due to moisture accumulation, such that a possibility of a tracking phenomenon due to an arc generation may be increased to deteriorate a long-term reliability.